1. Field of the Invention
The present invention relates in general to a control system for a plastic extruder and in particular to a control system for a plastic extruder which automatically controls both the pressure and the temperature of the extruded material.
2. Description of the Prior Art
Plastic extruders typically comprise a rotatable screw which is mounted in an extruder barrel having a hopper positioned at one end for receiving a plastic material such as a polymer in the form of solid pellets or chips. The rotatable screw functions to move the plastic material through the barrel which is typically divided into a plurality of heating zones. As the polymer is moved through the several heating zones of the barrel, the heated barrel walls and the frictional heat from the rotating screw cause the polymer to change from the solid state to the molten state. The molten material is then forced through a die which forms the material into the desired shape. The formed material can then be subjected to subsequent forming operations. For example, a polymer which is extruded in the shape of a tube can then be subjected to blow molding operations to form a plastic bottle.
When extruding certain materials, the temperatures along the extruder must be accurately controlled in accordance with properties of the particular polymer and of the extruder. Properties of the extruders include the type of feeder screw, the speed of the screw, the length of the barrel, etc. If the temperatures are not accurately controlled, the molten polymer will not be uniform and may decompose as a result of excessive temperatures.
One important operating parameter of an extruder is the melt temperature, the temperature of the molten material at the outlet of the barrel. The melt temperature is one indicator of the viscosity of the molten material and should remain constant for quality extrusions. Another important operating parameter of an extruder is the melt pressure, the pressure of the molten material at the outlet of the barrel. It is well known that, if the material composition remains the same, the output rate of the extruded material can be controlled by maintaining the melt temperature and melt pressure at constant values. The melt pressure is typically controlled by controlling the speed of the extruder screw while the melt temperature is controlled by regulating the temperature of each heating zone. Accurate control of both the melt temperature and the melt pressure is desirable as these parameters affect the quality and dimensional tolerances of the extruded material and the initial usage of raw material.
Initially, plastic extruders were controlled by manual adjustments. An extruder operator would attempt to manipulate extruder screw speed and extruder barrel temperatures such that a desired operating point could be achieved. An operating point of a plastic extruder can be defined as an amount of material output per unit time at a certain material temperature. Later, thermostatic controllers were utilized to help regulate barrel zone temperature. However, the quality of the extruded material of manually controlled extruders was highly dependent on the skill of the operator while thermostatic controllers did not give the high degree of control which was necessary for some materials.
Some of the disadvantages of manual or thermostatic control systems were overcome by an automatic temperature control system as disclosed in U.S. Pat. No. 3,698,844 to Erimm. In this system, separate temperature transducers sense the temperature of the material in each heating zone such that the temperature of each zone is maintained at a predetermined set point temperature. Another temperature transducer senses the melt temperature at the output of the barrel and generates a signal to an electronic controller which in turn adjusts the set points of each zone such that the sensed melt temperature equals a preselected desired melt temperature. However, in this system, if a temperature change in the barrel takes place as a result of a change in screw speed, this temperature change would not immediately be sensed by the melt temperature transducer. Thus, fluctuations in the melt temperature would occur as a result of the change in screw speed.
U.S. Pat. No. 3,733,059 to Pettit discloses an extruder temperature control system similar to the above-described Erimm system which provides accurate and prompt compensation for temperature changes in the barrel as a result of changes in screw speed. A tachometer generates a signal which varies as a function of the speed of the extruder screw. This signal is used in combination with conventional temperature set point signals to accurately maintain a constant melt temperature.
More recently, computer controlled systems have been utilized to control the operating parameters of an extruder. One such system is the CP-600 manufactured by Harrel, Inc., 16 Fitch Street, East Norwalk, Conn. 06855. This system is capable of closed loop feedback control of such process parameters as extrudate dimension, melt pressure and melt temperature. This system senses the melt temperature and automatically adjusts the barrel zone temperature such that melt temperature will remain at the desired temperature. A pressure transducer senses the melt pressure and a control loop maintains the desired melt pressure.
One of the problems associated with the prior art extruder control systems occurs in the design of the barrel zone temperature controllers. Preferably, these controllers are designed with a high sensitivity to disturbance signals. However, when a change in a temperature set point occurs, there is a danger in saturating the zone temperature controllers as the magnitude of the temperature set point changes are generally greater than the magnitude of disturbances. Hence, the sensitivity of the controller to disturbance signals must be reduced to prevent saturation of the controllers to set point changes.
Another problem associated with the prior art extruder control systems occurs as a result of the interaction between the melt temperature controller and the melt pressure controller. For example, a change in the melt temperature typically results in undesirable fluctuations in the melt pressure while a change in the melt pressure results in undesirable fluctuations in the melt temperature.